The introduction of low-cost interactive satellite terminals requires the development of multiple-access protocols capable of supporting a wide variety of traffic profiles characterizing different user classes. In particular, residential users generate a large volume of bursty traffic with a low utilization coefficient, for which demand assignment (DA) protocols routinely used in satellite communications systems are not very efficient because they introduce long transmission delays or inefficiencies linked to the signaling required by resource allocation systems. This problem is familiar to the person skilled in the art: on this topic see “Digital Video Broadcasting Return Channel via Satellite (DVB-RCS) Background Book”, 15 Nov. 2002, distributed by Nera Broadband Satellites AS, Bergerveien 12 PO Box 91 N-1375 Billingstad, Norway. Section 1.4.3 of the above work explains that the only access method that it is practical to use in such conditions is the slotted Aloha (SA) protocol, although this has only a low real efficiency of use of the capacity of the channel if the probability of loss of packets is to be acceptable, i.e, sufficiently low. Section 10.3.3.3 of “ETSI TR 101 790 v1.2.1 (2003-01)—Digital Video Broadcasting (DVB); Interaction Channel for Satellite Distribution Systems; Guidelines for the use of EN 301 790” also discloses the use of the Aloha protocol for the return channel in a satellite digital video broadcasting system. That document, and the ETSI document EN 301 790 V.1.3.1, March 2003, define standards for the interactive channel in the context of the satellite DVB standard, and are available from the European Telecommunications Standards Institute (ETSI), 650 Route des Lucioles, F-06921 Sophia Antipolis Cedex, France.
In practice, the Aloha protocol is essentially used for signaling and transmission capacity assignment request functions and sometimes for transmitting small data packets.
The Aloha protocol is a random access (RA) protocol developed in 1970: see N. Abramson, “The Aloha System—another Alternative for Computer Communications”, AFIPS Conf. Proc. Vol. 37, pp. 281-285, 1970. This basic principle is extremely simple: each user transmits data independently of the others and awaits an acknowledgement. If no acknowledgement is received, the user retransmits the same data with a random delay, and this procedure continues until the data is received correctly. Obviously, if two or more users transmit simultaneously, an access conflict arises, in other words a collision, which may entail the loss of the transmitted data (this is known as a “destructive” collision). For this reason, the system can function only if the utilization coefficient of each user (the fraction of the time actually used for transmission) is low. Statistical analysis shows that the maximum normalized bit rate that can be obtained is of the order of only 18%. That maximum bit rate is obtained when the number of attempts to transmit packets of unit duration per unit time is equal to 0.5.
The slotted Aloha (SA) protocol is a variant of the “pure” Aloha protocol that doubles the maximum bit rate compared to the above situation. A normalized bit rate of the order of 36% is therefore obtained when the number of attempts to transmit packets of unitary duration per unit time is equal to 1. This improvement in service is obtained by synchronizing users, dividing time into slots of predetermined duration (for example equal to the transmission time of a data packet), and transmitting the packets in corresponding relationship to said slots. On this topic see L. G. Roberts “ALOHA Packet Systems with and without Slots and Capture” ARPANET System note 8 (NIC11290), June 1972.
The paper by Gagan L. Chouldhury and Stephen S. Rappaport “Diversity ALOHA—A Random Access Scheme for Satellite Communications”, IEEE Transactions on Communications, vol. COM-31, No. 3, March 1983, describes an improvement to the SA protocol called the diversity slotted Aloha (DSA) protocol. The basic principle of that protocol is to transmit k>1 replicas of each packet, either with a random time shift (in TDMA systems), or simultaneously on channels of different frequency (in FDMA systems). That paper shows that by choosing the value of k appropriately it is possible to improve the performance of the SA protocol, as much in terms of capacity as of transmission delay, in particular if the system is not heavily loaded (fewer than one transmission attempt per unit time). Since the advantages of that variant are relatively limited, the SA protocol remains the most widely used protocol. One rare example of applications of the DSA protocol is the IPoS standard developed by the Telecommunication Industry Association (TIA), described in the document TIA-1008 “IP over Satellite”, October 2003.
In reality, for transmission delays and the packet loss rate to remain within acceptable limits, the protocols of the Aloha family must be used with a mean standardized loading (number of packets transmitted per time slot, or number of packets of unitary duration per unit time in the case of the pure SA protocol) of the order of only 2 to 5%, which gives mean bit rates much lower than the above-mentioned theoretical maximum values, with limited differences between the “pure” Aloha, SA and DSA protocols. On this topic see the paper by D. Raychaudhuri and K. Joseph “Channel Access Protocols for Ku-band VSAT networks: A Comparative Evaluation”, IEEE Comm. Magazine, Vol. 26, No. 5, pages 34-44, May 1998.